A hydrogen-generation assembly is an assembly that converts one or more feedstocks into a product stream containing hydrogen gas as a majority component. The produced hydrogen gas may be used in a variety of applications. One such application is energy production, such as in electrochemical fuel cells. An electrochemical fuel cell is a device that converts a fuel and an oxidant to electricity, a reaction product, and heat. For example, fuel cells may convert hydrogen and oxygen gases into water and electricity. In such fuel cells, the hydrogen gas is the fuel, the oxygen gas is the oxidant, and the water is the reaction product. Fuel cells typically require high purity hydrogen gas to prevent the fuel cells from being damaged during use. The product stream from a hydrogen-generation assembly may contain impurities, illustrative examples of which include one or more of carbon monoxide, carbon dioxide, methane, unreacted feedstock, and water. Therefore, there is a need in many conventional fuel cell systems to include suitable structure for removing impurities from the product hydrogen stream.
A pressure swing adsorption (PSA) process is an example of a mechanism that may be used to remove impurities from an impure hydrogen gas stream by selective adsorption of one or more of the impurities present in the impure hydrogen stream. The adsorbed impurities can be subsequently desorbed and removed from the PSA assembly. PSA is a pressure-driven separation process that utilizes a plurality of adsorbent beds. The beds are cycled through a series of steps, such as pressurization, separation (adsorption), depressurization (desorption), and purge steps to selectively remove impurities from the hydrogen gas and then desorb the impurities. A concern when using a PSA assembly is preventing breakthrough, which refers to when the adsorbent in a bed has been sufficiently saturated in adsorbed impurities that the impurities pass through the bed and thereby remain with the hydrogen gas instead of being retained in the bed. Conventionally, breakthrough prevention requires either intentional underperformance of the PSA assembly or the use of expensive composition-based detectors, such as carbon monoxide detectors, to determine when even a few parts per million (ppm) of carbon monoxide have passed through a bed. By “intentional underperformance,” it is meant that the PSA assembly is operated inefficiently, with each bed being used for impurity adsorption for only a subset of its capacity to provide a potentially wide margin of unused adsorbent and thereby hopefully prevent breakthrough. An advantage of such a process is that the cost and equipment required is reduced; however, the lack of actual breakthrough detection and the inefficient operation of the system may outweigh the cost and equipment savings, especially when it is realized that the composition of the stream to be purified may fluctuate due to malfunctions or other causes elsewhere in the hydrogen-generation assembly.